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Polyethylene terephthalate (or poly(ethylene terephthalate), PET, PETE, or the obsolete PETP or PET-P), is the most common
thermoplastic A thermoplastic, or thermosoft plastic, is any plastic polymer material that becomes pliable or moldable at a certain elevated temperature and solidifies upon cooling. Most thermoplastics have a high molecular weight. The polymer chains associate ...
polymer A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
resin of the
polyester Polyester is a category of polymers that contain the ester functional group in every repeat unit of their main chain. As a specific material, it most commonly refers to a type called polyethylene terephthalate (PET). Polyesters include natural ...
family and is used in fibres for clothing,
containers A container is any receptacle or enclosure for holding a product used in storage, packaging, and transportation, including shipping. Things kept inside of a container are protected on several sides by being inside of its structure. The term ...
for liquids and foods, and
thermoforming Thermoforming is a manufacturing process where a plastic sheet is heated to a pliable forming temperature, formed to a specific shape in a mold, and trimmed to create a usable product. The sheet, or "film" when referring to thinner gauges and cert ...
for manufacturing, and in combination with glass fibre for engineering
resins In polymer chemistry and materials science, resin is a solid or highly viscous substance of plant or synthetic origin that is typically convertible into polymers. Resins are usually mixtures of organic compounds. This article focuses on natu ...
. In 2016, annual production of PET was 56 million tons. The biggest application is in fibres (in excess of 60%), with bottle production accounting for about 30% of global demand. In the context of textile applications, PET is referred to by its common name,
polyester Polyester is a category of polymers that contain the ester functional group in every repeat unit of their main chain. As a specific material, it most commonly refers to a type called polyethylene terephthalate (PET). Polyesters include natural ...
, whereas the acronym ''PET'' is generally used in relation to packaging. Polyester makes up about 18% of world polymer production and is the fourth-most-produced polymer after
polyethylene Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most commonly produced plastic. It is a polymer, primarily used for packaging ( plastic bags, plastic films, geomembranes and containers including bo ...
(PE),
polypropylene Polypropylene (PP), also known as polypropene, is a thermoplastic polymer used in a wide variety of applications. It is produced via chain-growth polymerization from the monomer propylene. Polypropylene belongs to the group of polyolefins and ...
(PP) and polyvinyl chloride (PVC). PET consists of repeating (C10H8O4) units. PET is commonly
recycled Recycling is the process of converting waste materials into new materials and objects. The recovery of energy from waste materials is often included in this concept. The recyclability of a material depends on its ability to reacquire the p ...
, and has the digit 1 (♳) as its
resin identification code The ASTM International Resin Identification Coding System, often abbreviated RIC, is a set of symbols appearing on plastic products that identify the plastic resin out of which the product is made. It was developed in 1988 by the Society of t ...
(RIC). The National Association for PET Container Resources (NAPCOR) defines PET as: "Polyethylene terephthalate items referenced are derived from terephthalic acid (or
dimethyl terephthalate Dimethyl terephthalate (DMT) is an organic compound with the formula C6H4(COOCH3)2. It is the diester formed from terephthalic acid and methanol. It is a white solid that melts to give a distillable colourless liquid.Richard J. Sheehan "Terepht ...
) and mono
ethylene glycol Ethylene glycol (IUPAC name: ethane-1,2-diol) is an organic compound (a vicinal diol) with the formula . It is mainly used for two purposes, as a raw material in the manufacture of polyester fibers and for antifreeze formulations. It is an odo ...
, wherein the sum of terephthalic acid (or dimethyl terephthalate) and mono ethylene glycol reacted constitutes at least 90 percent of the mass of monomer reacted to form the polymer, and must exhibit a melting peak temperature between 225°C and 255°C, as identified during the second thermal scan in procedure 10.1 in ASTM D3418, when heating the sample at a rate of 10°C/minute." Depending on its processing and thermal history, polyethylene terephthalate may exist both as an amorphous (transparent) and as a
semi-crystalline polymer Crystallization of polymers is a process associated with partial alignment of their molecular chains. These chains fold together and form ordered regions called lamellae, which compose larger spheroidal structures named spherulites. Polymers can ...
. The semicrystalline material might appear transparent (particle size less than 500  nm) or opaque and white (particle size up to a few
micrometer Micrometer can mean: * Micrometer (device), used for accurate measurements by means of a calibrated screw * American spelling of micrometre The micrometre ( international spelling as used by the International Bureau of Weights and Measures; ...
s) depending on its crystal structure and particle size. One process for making PET uses bis(2-hydroxyethyl) terephthalate, which can be synthesized by the
esterification In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides ar ...
reaction between terephthalic acid and
ethylene glycol Ethylene glycol (IUPAC name: ethane-1,2-diol) is an organic compound (a vicinal diol) with the formula . It is mainly used for two purposes, as a raw material in the manufacture of polyester fibers and for antifreeze formulations. It is an odo ...
with water as a byproduct (this is also known as a condensation reaction), or by
transesterification In organic chemistry, transesterification is the process of exchanging the organic group R″ of an ester with the organic group R' of an alcohol. These reactions are often catalyzed by the addition of an acid or base catalyst. The reaction can ...
reaction between
ethylene glycol Ethylene glycol (IUPAC name: ethane-1,2-diol) is an organic compound (a vicinal diol) with the formula . It is mainly used for two purposes, as a raw material in the manufacture of polyester fibers and for antifreeze formulations. It is an odo ...
and
dimethyl terephthalate Dimethyl terephthalate (DMT) is an organic compound with the formula C6H4(COOCH3)2. It is the diester formed from terephthalic acid and methanol. It is a white solid that melts to give a distillable colourless liquid.Richard J. Sheehan "Terepht ...
(DMT) with
methanol Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical and the simplest aliphatic alcohol, with the formula C H3 O H (a methyl group linked to a hydroxyl group, often abbreviated as MeOH). It is a ...
as a byproduct. Polymerization is through a
polycondensation In polymer chemistry, condensation polymers are any kind of polymers whose process of polymerization involves a condensation reaction (i.e. a small molecule, such as water or methanol, is produced as a byproduct). Condensation polymers are for ...
reaction of the monomers (done immediately after esterification/transesterification) with water as the byproduct. :


Uses

Plastic-recyc-01.svg, PET has SPI resin ID code 1 File:PETling.jpg, PET preform for injection stretch blow moulding of a bottle File:Clean the Bay 2012 (7324648864).jpg, A finished PET bottle File:Pet plastic crystallisation.jpg, A PET bottle which has been heated by a candle and has recrystallized, making it opaque. File:PET-Verpackung-offen.jpg, PET clamshell packaging, used to sell fruit, hardware, etc. File:Buso de Algodon y Poliester.JPG, Polyester yarn Mikrofaser-Handtuch für Unterwegs.JPG, Microfiber towels and cleaning cloths File:Mylar balloons.jpg, Aluminized Mylar balloons filled with
helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...


Textiles

Polyester fibres are widely used in the textile industry. The invention of the polyester fibre is attributed to J. R. Whinfield. It was first commercialized in the 1940s by ICI, under the brand 'Terylene'. Subsequently E. I. DuPont launched the brand 'Dacron'. As of 2022, there are many brands around the world, mostly Asian. Polyester fibres are used in fashion apparel often blended with cotton, as heat insulation layers in thermal wear, sportswear and workwear and automotive upholstery.


Rigid packaging

Plastic bottles made from PET are widely used for
soft drink A soft drink (see § Terminology for other names) is a drink that usually contains water (often carbonated), a sweetener, and a natural and/or artificial flavoring. The sweetener may be a sugar, high-fructose corn syrup, fruit juice, a su ...
s, both still and sparkling. For beverages that are degraded by oxygen, such as beer, a multilayer structure is used. PET sandwiches an additional
polyvinyl alcohol Poly(vinyl alcohol) (PVOH, PVA, or PVAl) is a water-soluble synthetic polymer. It has the idealized formula H2CH(OH)sub>''n''. It is used in papermaking, textile warp sizing, as a thickener and emulsion stabilizer in polyvinyl acetate (PVAc) ...
(PVOH) or
polyamide A polyamide is a polymer with repeating units linked by amide bonds. Polyamides occur both naturally and artificially. Examples of naturally occurring polyamides are proteins, such as wool and silk. Artificially made polyamides can be made through ...
(PA) layer to further reduce its oxygen permeability. Non-oriented PET sheet can be
thermoformed Thermoforming is a manufacturing process where a plastic sheet is heated to a pliable forming temperature, formed to a specific shape in a mold, and trimmed to create a usable product. The sheet, or "film" when referring to thinner gauges and cert ...
to make packaging trays and
blister packs A blister pack is any of several types of pre-formed plastic packaging used for small consumer goods, foods, and for pharmaceuticals. The primary component of a blister pack is a cavity or pocket made from a formable web, usually a thermofor ...
. Crystallizable PET withstands freezing and oven baking temperatures. Both amorphous PET and BoPET are transparent to the naked eye. Color-conferring dyes can easily be formulated into PET sheet. PET is permeable to oxygen and carbon dioxide and this imposes shelf life limitations of contents packaged in PET.


Flexible packaging

Biaxially oriented PET (BOPET) film (often known by one of its trade names, "Mylar") can be aluminized by
evaporating Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when humidi ...
a
thin film A thin film is a layer of material ranging from fractions of a nanometer (monolayer) to several micrometers in thickness. The controlled synthesis of materials as thin films (a process referred to as deposition) is a fundamental step in many ap ...
of metal onto it to reduce its permeability, and to make it reflective and opaque ( MPET). These properties are useful in many applications, including flexible food
packaging Packaging is the science, art and technology of enclosing or protecting products for distribution, storage, sale, and use. Packaging also refers to the process of designing, evaluating, and producing packages. Packaging can be described as a co ...
and
thermal insulation Thermal insulation is the reduction of heat transfer (i.e., the transfer of thermal energy between objects of differing temperature) between objects in thermal contact or in range of radiative influence. Thermal insulation can be achieved with s ...
(such as
space blanket A space blanket (also known as a Mylar blanket, emergency blanket, first aid blanket, safety blanket, thermal blanket, weather blanket, heat sheet, foil blanket, or shock blanket) is an especially low-weight, low-bulk blanket made of heat-refle ...
s).


Photovoltaic modules

BOPET is used in the backsheet of
photovoltaic module Photovoltaics (PV) is the conversion of light into electricity using semiconducting materials that exhibit the photovoltaic effect, a phenomenon studied in physics, photochemistry, and electrochemistry. The photovoltaic effect is commercially ...
s. Most backsheets consist of a layer of BOPET laminated to a fluoropolymer or a layer of UV stabilized BOPET. PET is also used as a substrate in thin film solar cells.


Thermoplastic resins

PET can be compounded with glass fibre and crystallization accelerators, to make thermoplastic resins. These can be injection moulded into parts such as housings, covers, electrical appliance components and elements of the ignition system.


Other applications

* A waterproofing barrier in undersea cables. * As a fibre, spliced into bell rope tops to help prevent wear on the ropes as they pass through the ceiling. * Since late 2014 as liner material in type IV composite high pressure gas cylinders. PET works as a much better barrier to oxygen than earlier used (LD)PE. * As a
3D printing 3D printing or additive manufacturing is the Manufacturing, construction of a three-dimensional object from a computer-aided design, CAD model or a digital 3D modeling, 3D model. It can be done in a variety of processes in which material is ...
filament, as well as in the 3D printing plastic
PETG Polyethylene terephthalate (or poly(ethylene terephthalate), PET, PETE, or the obsolete PETP or PET-P), is the most common thermoplastic polymer resin of the polyester family and is used in fibres for clothing, containers for liquids and foods, ...
. * Film for tape applications, such as the carrier for
magnetic tape Magnetic tape is a medium for magnetic storage made of a thin, magnetizable coating on a long, narrow strip of plastic film. It was developed in Germany in 1928, based on the earlier magnetic wire recording from Denmark. Devices that use magne ...
or backing for pressure-sensitive adhesive tapes. Digitalization has caused the virtual disappeance of the magnetic audio and videotape application. * Water-resistant paper.


History

PET was patented in 1941 by
John Rex Whinfield John Rex Whinfield CBE (16 February 1901 in Sutton, Surrey, England – 6 July 1966 in Dorking, Surrey) was a British chemist. Together with James Tennant Dickson, Whinfield investigated polyesters and produced and patented the first polyeste ...
, James Tennant Dickson and their employer the
Calico Printers' Association The Calico Printers' Association Ltd was a British textile company founded in 1899, from the amalgamation of 46 textile printing companies and 13 textile merchants. The industry had prospered in the latter half of the 19th century but the fierce ...
of
Manchester Manchester () is a city in Greater Manchester, England. It had a population of 552,000 in 2021. It is bordered by the Cheshire Plain to the south, the Pennines to the north and east, and the neighbouring city of Salford to the west. The t ...
, England. E. I. DuPont de Nemours in Delaware, United States, first used the trademark Mylar in June 1951 and received registration of it in 1952. It is still the best-known name used for polyester film. The current owner of the trademark is DuPont Teijin Films. In the Soviet Union, PET was first manufactured in the laboratories of the Institute of High-Molecular Compounds of the
USSR Academy of Sciences The Academy of Sciences of the Soviet Union was the highest scientific institution of the Soviet Union from 1925 to 1991, uniting the country's leading scientists, subordinated directly to the Council of Ministers of the Soviet Union (until 1946 ...
in 1949, and its name "Lavsan" is an acronym thereof (лаборатории Института высокомолекулярных соединений Академии наук СССР). The PET bottle was invented in 1973 by Nathaniel Wyeth and patented by DuPont.


Physical properties

PET in its most stable state is a colorless, semi-crystalline resin. However it is intrinsically slow to crystallize compared to other semicrystalline polymers. Depending on processing conditions it can be formed into either amorphous or crystalline articles. Its amenability to drawing makes PET useful in fibre and film applications. Like most aromatic polymers, it has better barrier properties than aliphatic polymers. It is strong and impact-resistant. PET is
hygroscopic Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substance ...
. About 60% crystallization is the upper limit for commercial products, with the exception of polyester fibers. Transparent products can be produced by rapidly cooling molten polymer below Tg
glass transition temperature The glass–liquid transition, or glass transition, is the gradual and reversible transition in amorphous materials (or in amorphous regions within semicrystalline materials) from a hard and relatively brittle "glassy" state into a viscous or rubb ...
to form an
amorphous solid In condensed matter physics and materials science, an amorphous solid (or non-crystalline solid, glassy solid) is a solid that lacks the long-range order that is characteristic of a crystal. Etymology The term comes from the Greek ''a'' ("wit ...
. Like glass, amorphous PET forms when its molecules are not given enough time to arrange themselves in an orderly, crystalline fashion as the melt is cooled. At room temperature the molecules are frozen in place, but, if enough heat energy is put back into them by heating above Tg, they begin to move again, allowing crystals to
nucleate In thermodynamics, nucleation is the first step in the formation of either a new thermodynamic phase or structure via self-assembly or self-organization within a substance or mixture. Nucleation is typically defined to be the process that dete ...
and grow. This procedure is known as solid-state crystallization. When allowed to cool slowly, the molten polymer forms a more crystalline material. This material has
spherulites In petrology, spherulites () are small, rounded bodies that commonly occur in vitreous igneous rocks. They are often visible in specimens of obsidian, pitchstone, and rhyolite as globules about the size of millet seed or rice grain, with a dul ...
containing many small
crystallite A crystallite is a small or even microscopic crystal which forms, for example, during the cooling of many materials. Crystallites are also referred to as grains. Bacillite is a type of crystallite. It is rodlike with parallel longulites. Stru ...
s when crystallized from an amorphous solid, rather than forming one large single crystal. Light tends to scatter as it crosses the boundaries between crystallites and the amorphous regions between them, causing the resulting solid to be translucent. Orientation also renders polymers more transparent. This is why BOPET film and bottles are both crystalline to a degree and transparent. Amorphous PET crystallizes and becomes opaque when exposed to solvents such as chloroform or toluene. PET is stoichiometrically a mixture of carbon and , and therefore has been used in an experiment involving laser-driven shock compression which created
nanodiamond Nanodiamonds, or diamond nanoparticles, are diamonds with a size below 100 nanometers. They can be produced by impact events such as an explosion or meteoritic impacts. Because of their inexpensive, large-scale synthesis, potential for surfa ...
s and
superionic water Superionic water, also called superionic ice or ice XVIII is a phase of water that exists at extremely high temperatures and pressures. In superionic water, water molecules break apart and the oxygen ions crystallize into an evenly spaced lat ...
. This could be a possible way of producing nanodiamonds commercially.


Absorption/scalping

PET has an affinity for hydrophobic flavors and drinks sometimes need to be formulated with higher dosage compared to glass to offset the flavor taken up by the container.Heavy gauge PET bottles are sometimes returnable for re-use and is practiced in some EU countries, however the propensity of PET to absorb flavors makes it necessary to conduct a "sniffer" test on returned bottles to avoid cross-contamination.


Intrinsic viscosity

Different applications of PET require different degrees of polymerization, which can be obtained by modifying the process conditions. The
molecular weight A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...
of PET is measured by solution viscosity. The preferred method is
intrinsic viscosity Intrinsic viscosity \left \eta \right/math> is a measure of a solute's contribution to the viscosity \eta of a solution. It should not be confused with inherent viscosity, which is the ratio of the natural logarithm of the relative viscosity to th ...
(IV). IV is a dimensionless measurement. It is found by extrapolating the relative viscosity (measured in (dℓ/g)) to zero concentration. Shown below are the IV ranges for the main applications: ; Fibers :* 0.40–0.70: textile :* 0.72–0.98: technical eg
tire A tire (American English) or tyre (British English) is a ring-shaped component that surrounds a Rim (wheel), wheel's rim to transfer a vehicle's load from the axle through the wheel to the ground and to provide Traction (engineering), t ...
cord ; Films :* 0.60–0.70: biaxially oriented PET film :* 0.70–1.00: sheet grade for
thermoforming Thermoforming is a manufacturing process where a plastic sheet is heated to a pliable forming temperature, formed to a specific shape in a mold, and trimmed to create a usable product. The sheet, or "film" when referring to thinner gauges and cert ...
; Bottles :* 0.70–0.78: general purpose bottles :* 0.78–0.85: bottles for carbonated drinks ; Monofilaments,
engineering plastic Engineering plastics are a group of plastic materials that have better mechanical and/or thermal properties than the more widely used commodity plastics (such as polystyrene, PVC, polypropylene and polyethylene). Being more expensive than s ...
s: :* 1.00–2.00


Copolymers

PET is copolymerized with other diols or diacids to optimize the properties for particular applications. For example,
cyclohexanedimethanol Cyclohexanedimethanol (CHDM) is a mixture of isomeric organic compounds with formula C6H10(CH2OH)2. It is a colorless low-melting solid used in the production of polyester resins. Commercial samples consist of a mixture of cis and trans isomers. ...
(CHDM) can be added to the polymer backbone in place of
ethylene glycol Ethylene glycol (IUPAC name: ethane-1,2-diol) is an organic compound (a vicinal diol) with the formula . It is mainly used for two purposes, as a raw material in the manufacture of polyester fibers and for antifreeze formulations. It is an odo ...
. Since this building block is much larger (six additional carbon atoms) than the ethylene glycol unit it replaces, it does not fit in with the neighboring chains the way an ethylene glycol unit would. This interferes with crystallization and lowers the polymer's melting temperature. In general, such PET is known as PETG or PET-G (polyethylene terephthalate glycol-modified). It is a clear amorphous thermoplastic that can be injection-molded, sheet-extruded or extruded as filament for
3D printing 3D printing or additive manufacturing is the Manufacturing, construction of a three-dimensional object from a computer-aided design, CAD model or a digital 3D modeling, 3D model. It can be done in a variety of processes in which material is ...
. PETG can be colored during processing. Another common modifier is
isophthalic acid Isophthalic acid is an organic compound with the formula C6H4(CO2H)2. This colorless solid is an isomer of phthalic acid and terephthalic acid. The main industrial uses of purified isophthalic acid (PIA) are for the production of polyethylene te ...
, replacing some of the 1,4-(''para-'') linked
terephthalate Terephthalic acid is an organic compound with formula C6H4(CO2H)2. This white solid is a commodity chemical, used principally as a precursor to the polyester PET, used to make clothing and plastic bottles. Several million tonnes are produced annua ...
units. The 1,2-(''ortho-'') or 1,3-('' meta''-) linkage produces an angle in the chain, which also disturbs crystallinity. Such copolymers are advantageous for certain molding applications, such as
thermoforming Thermoforming is a manufacturing process where a plastic sheet is heated to a pliable forming temperature, formed to a specific shape in a mold, and trimmed to create a usable product. The sheet, or "film" when referring to thinner gauges and cert ...
, which is used for example to make tray or blister packaging from co-PET film, or amorphous PET sheet (A-PET/PETA) or PETG sheet. On the other hand, crystallization is important in other applications where mechanical and dimensional stability are important, such as seat belts. For PET bottles, the use of small amounts of isophthalic acid, CHDM,
diethylene glycol Diethylene glycol (DEG) is an organic compound with the formula (HOCH2CH2)2O. It is a colorless, practically odorless, and hygroscopic liquid with a sweetish taste. It is a four carbon dimer of ethylene glycol. It is miscible in water, alcohol, ...
(DEG) or other comonomers can be useful: if only small amounts of comonomers are used, crystallization is slowed but not prevented entirely. As a result, bottles are obtainable via
stretch blow molding Blow molding (or moulding) is a manufacturing process for forming hollow plastic parts. It is also used for forming glass bottles or other hollow shapes. In general, there are three main types of blow molding: extrusion blow molding, injection ...
("SBM"), which are both clear and crystalline enough to be an adequate barrier to aromas and even gases, such as carbon dioxide in carbonated beverages.


Production

Polyethylene terephthalate is produced from
ethylene glycol Ethylene glycol (IUPAC name: ethane-1,2-diol) is an organic compound (a vicinal diol) with the formula . It is mainly used for two purposes, as a raw material in the manufacture of polyester fibers and for antifreeze formulations. It is an odo ...
(usually referred to in the trade as "MEG", for monoethylene glycol) and
dimethyl terephthalate Dimethyl terephthalate (DMT) is an organic compound with the formula C6H4(COOCH3)2. It is the diester formed from terephthalic acid and methanol. It is a white solid that melts to give a distillable colourless liquid.Richard J. Sheehan "Terepht ...
(DMT) (C6H4(CO2CH3)2) but mostly terephthalic acid (known in the trade as "PTA", for purified terephthalic acid). As of 2022, ethylene glycol is made from
ethene Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon-carbon double bonds). Ethylene i ...
found in
natural gas Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbo ...
, while terephthalic acid comes from
p-xylene ''p''-Xylene ( ''para''-xylene) is an aromatic hydrocarbon. It is one of the three isomers of dimethylbenzene known collectively as xylenes. The ''p-'' stands for ''para-'', indicating that the two methyl groups in ''p''-xylene occupy the diamet ...
made from
crude oil Petroleum, also known as crude oil, or simply oil, is a naturally occurring yellowish-black liquid mixture of mainly hydrocarbons, and is found in geological formations. The name ''petroleum'' covers both naturally occurring unprocessed crude ...
. Typically an antimony or titanium compound is used as a catalyst, a phosphite is added as a stabilizer and a bluing agent such as cobalt salt is added to mask any yellowing.


Dimethyl terephthalate (DMT) process

In the
dimethyl terephthalate Dimethyl terephthalate (DMT) is an organic compound with the formula C6H4(COOCH3)2. It is the diester formed from terephthalic acid and methanol. It is a white solid that melts to give a distillable colourless liquid.Richard J. Sheehan "Terepht ...
(DMT) process, DMT and excess MEG are
transesterified In organic chemistry, transesterification is the process of exchanging the organic group R″ of an ester with the organic group R' of an alcohol. These reactions are often catalyzed by the addition of an acid or base catalyst. The reaction ca ...
in the melt at 150–200 °C with a basic catalyst.
Methanol Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical and the simplest aliphatic alcohol, with the formula C H3 O H (a methyl group linked to a hydroxyl group, often abbreviated as MeOH). It is a ...
(CH3OH) is removed by distillation to drive the reaction forward. Excess MEG is distilled off at higher temperature with the aid of vacuum. The second transesterification step proceeds at 270–280 °C, with continuous distillation of MEG as well. The reactions can be summarized as follows: ;First step : C6H4(CO2CH3)2 + 2 HOCH2CH2OH → C6H4(CO2CH2CH2OH)2 + 2 CH3OH ;Second step : ''n'' C6H4(CO2CH2CH2OH)2CO)C6H4(CO2CH2CH2O)sub>n + ''n'' HOCH2CH2OH


Terephthalic acid (PTA) process

In the terephthalic acid process, MEG and PTA are esterified directly at moderate pressure (2.7–5.5 bar) and high temperature (220–260 °C). Water is eliminated in the reaction, and it is also continuously removed by distillation: : ''n'' C6H4(CO2H)2 + ''n'' HOCH2CH2OH → CO)C6H4(CO2CH2CH2O)sub>n + 2''n'' H2O


Bio-PET

Bio-PET is the bio-based counterpart of PET. Essentially in Bio-PET, the MEG is manufactured from ethylene derived from sugar cane
ethanol Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an Alcohol (chemistry), alcohol with the chemical formula . Its formula can be also written as or (an ethyl ...
. A better process based on oxidation of ethanol has been proposed, and it is also technically possible to make PTA from readily available biobased furfural.


Degradation

PET is subject to
degradation Degradation may refer to: Science * Degradation (geology), lowering of a fluvial surface by erosion * Degradation (telecommunications), of an electronic signal * Biodegradation of organic substances by living organisms * Environmental degradatio ...
during processing. If the moisture level is too high, hydrolysis will reduce the molecular weight by chain scission, resulting in brittleness. If the residence time and/or melt temperature are too high, then thermal degradation or thermooxidative degradation will occur resulting in: *discoloration *reduced molecular weight *formation of
acetaldehyde Acetaldehyde (IUPAC systematic name ethanal) is an organic chemical compound with the formula CH3 CHO, sometimes abbreviated by chemists as MeCHO (Me = methyl). It is a colorless liquid or gas, boiling near room temperature. It is one of the mos ...
, *
cross-link In chemistry and biology a cross-link is a bond or a short sequence of bonds that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers can be either synthetic polymers or natural ...
ing ("gel" or "fish-eye" formation). Mitigation measures include *
copolymerisation In polymer chemistry, a copolymer is a polymer derived from more than one species of monomer. The polymerization of monomers into copolymers is called copolymerization. Copolymers obtained from the copolymerization of two monomer species are some ...
. Comonomers such as CHDM or
isophthalic acid Isophthalic acid is an organic compound with the formula C6H4(CO2H)2. This colorless solid is an isomer of phthalic acid and terephthalic acid. The main industrial uses of purified isophthalic acid (PIA) are for the production of polyethylene te ...
lower the melting point and thus the melt temperature of the resin (
copolymers In polymer chemistry, a copolymer is a polymer derived from more than one species of monomer. The polymerization of monomers into copolymers is called copolymerization. Copolymers obtained from the copolymerization of two monomer species are some ...
, above). *The addition of polymer stabilisers such as
phosphites A phosphite anion or phosphite in inorganic chemistry usually refers to PO3sup>2− but includes 2PO3sup>− ( PO2(OH)sup>−). These anions are the conjugate bases of phosphorous acid (H3PO3). The corresponding salts, e.g. sodium phosphite ...
.


Acetaldehyde

Acetaldehyde Acetaldehyde (IUPAC systematic name ethanal) is an organic chemical compound with the formula CH3 CHO, sometimes abbreviated by chemists as MeCHO (Me = methyl). It is a colorless liquid or gas, boiling near room temperature. It is one of the mos ...
is a colorless, volatile substance with a fruity smell. Although it forms naturally in some fruit, it can cause an off-taste in bottled water. Acetaldehyde forms by degradation of PET through the mishandling of the material. High temperatures (PET decomposes above 300 °C or 570 °F), high pressures, extruder speeds (excessive shear flow raises temperature), and long barrel residence times all contribute to the production of acetaldehyde.
Photo-oxidation In polymer chemistry photo-oxidation (sometimes: oxidative photodegradation) is the degradation of a polymer surface due to the combined action of light and oxygen. It is the most significant factor in the weathering of plastics. Photo-oxidatio ...
can also cause the gradual formation acetaldehyde over the object's lifespan. This proceeds via a Type II
Norrish reaction A Norrish reaction in organic chemistry is a photochemical reaction taking place with ketones and aldehydes. Such reactions are subdivided into Norrish type I reactions and Norrish type II reactions. The reaction is named after Ronald George Wreyfo ...
. When acetaldehyde is produced, some of it remains dissolved in the walls of a container and then diffuses into the product stored inside, altering the taste and aroma. This is not such a problem for non-consumables (such as shampoo), for fruit juices (which already contain acetaldehyde), or for strong-tasting drinks like soft drinks. For bottled water, however, low acetaldehyde content is quite important, because, if nothing masks the aroma, even extremely low concentrations (10–20 parts per billion in the water) of acetaldehyde can produce an off-taste.


Biodegradation

At least one species of bacterium in the genus '' Nocardia'' can degrade PET with an
esterase An esterase is a hydrolase enzyme that splits esters into an acid and an alcohol in a chemical reaction with water called hydrolysis. A wide range of different esterases exist that differ in their substrate specificity, their protein structure ...
enzyme. Esterases are enzymes able to cleave the ester bond. Also, the initial degradation of PET can be esterases expressed by ''Bacillus'' and ''Nocardia''. Japanese scientists have isolated a bacterium ''
Ideonella sakaiensis ''Ideonella sakaiensis'' is a bacterium from the genus'' Ideonella'' and family Comamonadaceae capable of breaking down and consuming the plastic polyethylene terephthalate (PET) using it as both a carbon and energy source. The bacterium was ...
'' that possesses two enzymes which can break down the PET into smaller pieces that the bacterium can digest. A colony of ''I. sakaiensis'' can disintegrate a plastic film in about six weeks. An enzyme based on a natural PET-ase was designed with the help of a machine learning algorithm to be able to tolerate pH and temperature changes by the
University of Texas at Austin The University of Texas at Austin (UT Austin, UT, or Texas) is a public research university in Austin, Texas. It was founded in 1883 and is the oldest institution in the University of Texas System. With 40,916 undergraduate students, 11,075 ...
. The PET-ase was found to able to degrade various products and could break them down as fast as 24 hours.


Environmental concerns


Resource depletion

Compared to the use of petroleum as fuel, however, the amount of crude oil processed into PET is very small. The total production capacity of PET is around 30 million tons, compared to 4.2 billion tons of crude oil production, thus around 0.7% of crude oil is processed into PET.


End of life


Recycle

PET bottles lend themselves well to recycling (see below). In many countries PET bottles are recycled to a substantial degree, for example about 75% in Switzerland. The term rPET is commonly used to describe the recycled material, though it is also referred to as R-PET or post-consumer PET (POSTC-PET).


Energy recovery

PET is a desirable fuel for
Waste-to-energy plant A waste-to-energy plant is a waste management facility that combusts wastes to produce electricity. This type of power plant is sometimes called a trash-to-energy, municipal waste incineration, energy recovery, or resource recovery plant. Modern ...
s, as it has a high calorific value which helps to reduce the use of primary resources for energy generation.


Littering

Nevertheless, littering has become a prominent issue in public opinion, and PET bottles are a visible part of that.


Dumping of apparel

A substantial amount of post consumer waste from the textile industry ends up in landfills in developing countries such as Chile and in countries in West Africa such as Ghana. PET being a substantial component of apparel, this waste in landfills contains much PET.


Microfibres from apparel and microplastics

Clothing sheds microfibres in use, during washing and machine drying. Plastic litter slowly forms small particles. Microplastics which are present on the bottom of the river or seabed can ingested by small marine life, thus entering the food chain. As PET has a higher density than water, a significant amount of PET microparticles may be precipitated in sewage treatment plants. PET microfibers generated by apparel wear, washing or machine drying can become airborne, and be dispersed into fields, where they are ingested by livestock or plants and end up in the human food supply.
SAPEA The Scientific Advice Mechanism is a service created by the European Commission which provides independent science advice directly to European Commissioners to inform their decision-making. The Mechanism consists of two parts: the Group of Chief ...
have declared that such particles 'do not pose a widespread risk'. PET is known to degrade when exposed to sunlight and oxygen. As of 2016, scarce information exists regarding the life-time of the synthetic polymers in the environment.


Safety

Commentary published in ''
Environmental Health Perspectives ''Environmental Health Perspectives'' (''EHP'') is a peer-reviewed Peer review is the evaluation of work by one or more people with similar competencies as the producers of the work (peers). It functions as a form of self-regulation by qua ...
'' in April 2010 suggested that PET might yield
endocrine disruptor Endocrine disruptors, sometimes also referred to as hormonally active agents, endocrine disrupting chemicals, or endocrine disrupting compounds are chemicals that can interfere with endocrine (or hormonal) systems. These disruptions can cause c ...
s under conditions of common use and recommended research on this topic. Proposed mechanisms include leaching of
phthalates Phthalates (, ), or phthalate esters, are esters of phthalic acid. They are mainly used as plasticizers, i.e., substances added to plastics to increase their flexibility, transparency, durability, and longevity. They are used primarily to soften ...
as well as leaching of
antimony Antimony is a chemical element with the symbol Sb (from la, stibium) and atomic number 51. A lustrous gray metalloid, it is found in nature mainly as the sulfide mineral stibnite (Sb2S3). Antimony compounds have been known since ancient time ...
. An article published in ''
Journal of Environmental Monitoring ''Environmental Science: Processes & Impacts'' is a monthly peer-reviewed scientific journal covering all aspects of environmental science. It is published by the Royal Society of Chemistry and Kris McNeill is the editor-in-chief. The journal wa ...
'' in April 2012 concludes that antimony concentration in
deionized water Purified water is water that has been mechanically filtered or processed to remove impurities and make it suitable for use. Distilled water was, formerly, the most common form of purified water, but, in recent years, water is more frequently puri ...
stored in PET bottles stays within EU's acceptable limit even if stored briefly at temperatures up to 60 °C (140 °F), while bottled contents (water or soft drinks) may occasionally exceed the EU limit after less than a year of storage at room temperature.


Antimony

Antimony Antimony is a chemical element with the symbol Sb (from la, stibium) and atomic number 51. A lustrous gray metalloid, it is found in nature mainly as the sulfide mineral stibnite (Sb2S3). Antimony compounds have been known since ancient time ...
(Sb) is a
metalloid A metalloid is a type of chemical element which has a preponderance of material property, properties in between, or that are a mixture of, those of metals and nonmetals. There is no standard definition of a metalloid and no complete agreement on ...
element that is used as a
catalyst Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
in the form of compounds such as antimony trioxide (Sb2O3) or antimony triacetate in the production of PET. After manufacturing, a detectable amount of antimony can be found on the surface of the product. This residue can be removed with washing. Antimony also remains in the material itself and can, thus, migrate out into food and drinks. Exposing PET to boiling or microwaving can increase the levels of antimony significantly, possibly above US EPA maximum contamination levels. The drinking water limit assessed by WHO is 20 parts per billion (WHO, 2003), and the drinking water limit in the United States is 6 parts per billion. Although antimony trioxide is of low toxicity when taken orally,Guidelines for drinking – water quality
who.int
its presence is still of concern. The Swiss
Federal Office of Public Health The Federal Office of Public Health (FOPH) is the Swiss federal government’s centre for public health and a part of the Swiss Federal Department of Home Affairs. In addition to developing national health policy, it also represents the inte ...
investigated the amount of antimony migration, comparing waters bottled in PET and glass: The antimony concentrations of the water in PET bottles were higher, but still well below the allowed maximum concentration. The Swiss Federal Office of Public Health concluded that small amounts of antimony migrate from the PET into bottled water, but that the health risk of the resulting low concentrations is negligible (1% of the "
tolerable daily intake Tolerable daily intake (TDI) refers to the daily amount of a chemical that has been assessed safe for human being on long-term basis (usually whole lifetime). Originally acceptable daily intake (ADI) was introduced in 1961 to define the daily intake ...
" determined by the
WHO Who or WHO may refer to: * Who (pronoun), an interrogative or relative pronoun * Who?, one of the Five Ws in journalism * World Health Organization Arts and entertainment Fictional characters * Who, a creature in the Dr. Seuss book '' Horton He ...
). A later (2006) but more widely publicized study found similar amounts of antimony in water in PET bottles. The WHO has published a risk assessment for antimony in drinking water. Fruit juice concentrates (for which no guidelines are established), however, that were produced and bottled in PET in the UK were found to contain up to 44.7 μg/L of antimony, well above the EU limits for
tap water Tap water (also known as faucet water, running water, or municipal water) is water supplied through a tap, a water dispenser valve. In many countries, tap water usually has the quality of drinking water. Tap water is commonly used for drinking, ...
of 5 μg/L.


Bottle processing equipment

There are two basic molding methods for PET bottles, one-step and two-step. In two-step molding, two separate machines are used. The first machine injection molds the preform, which resembles a test tube, with the bottle-cap threads already molded into place. The body of the tube is significantly thicker, as it will be inflated into its final shape in the second step using
stretch blow molding Blow molding (or moulding) is a manufacturing process for forming hollow plastic parts. It is also used for forming glass bottles or other hollow shapes. In general, there are three main types of blow molding: extrusion blow molding, injection ...
. In the second step, the preforms are heated rapidly and then inflated against a two-part mold to form them into the final shape of the bottle. Preforms (uninflated bottles) are now also used as robust and unique containers themselves; besides novelty candy, some
Red Cross The International Red Cross and Red Crescent Movement is a Humanitarianism, humanitarian movement with approximately 97 million Volunteering, volunteers, members and staff worldwide. It was founded to protect human life and health, to ensure re ...
chapters distribute them as part of the
Vial of Life The Vial of Life (french: Fiole de vie) (aka Vial of L.I.F.E (Lifesaving Information for Emergencies)) is a program that allows individuals to have their complete medical information ready in their home for Emergency medical services, emergency ...
program to homeowners to store medical history for emergency responders. In one-step machines, the entire process from raw material to finished container is conducted within one machine, making it especially suitable for molding non-standard shapes (custom molding), including jars, flat oval, flask shapes, etc. Its greatest merit is the reduction in space, product handling and energy, and far higher visual quality than can be achieved by the two-step system.


Polyester recycling industry

While most thermoplastics can, in principle, be recycled,
PET bottle recycling Although PET is used in several applications, (principally textile fibres for apparel and upholstery, bottles and other rigid packaging, flexible packaging and electrical and electronic goods), as of 2022 only bottles are collected at a subst ...
is more practical than many other plastic applications because of the high value of the resin and the almost exclusive use of PET for widely used water and carbonated soft drink bottling. PET has a
resin identification code The ASTM International Resin Identification Coding System, often abbreviated RIC, is a set of symbols appearing on plastic products that identify the plastic resin out of which the product is made. It was developed in 1988 by the Society of t ...
of 1. The prime uses for recycled PET are polyester
fiber Fiber or fibre (from la, fibra, links=no) is a natural or artificial substance that is significantly longer than it is wide. Fibers are often used in the manufacture of other materials. The strongest engineering materials often incorporate ...
, strapping, and non-food containers. Because of the recyclability of PET and the relative abundance of
post-consumer waste Post-consumer waste is a waste type produced by the end consumer of a material stream; that is, where the waste-producing use did not involve the production of another product. The terms of pre-consumer and post-consumer recycled materials are ...
in the form of bottles, PET is rapidly gaining market share as a carpet fiber.
Mohawk Industries Mohawk Industries is an American flooring manufacturer based in Calhoun, Georgia, United States. Mohawk produces floor covering products for residential and commercial applications in North America and residential applications in Europe. The com ...
released everSTRAND in 1999, a 100% post-consumer recycled content PET fiber. Since that time, more than 17 billion bottles have been recycled into carpet fiber. Pharr Yarns, a supplier to numerous carpet manufacturers including Looptex, Dobbs Mills, and Berkshire Flooring, produces a BCF (bulk continuous filament) PET carpet fiber containing a minimum of 25% post-consumer recycled content. PET, like many plastics, is also an excellent candidate for thermal disposal (
incineration Incineration is a waste treatment process that involves the combustion of substances contained in waste materials. Industrial plants for waste incineration are commonly referred to as waste-to-energy facilities. Incineration and other high ...
), as it is composed of carbon, hydrogen, and oxygen, with only trace amounts of catalyst elements (but no sulfur). PET has the energy content of soft coal. When recycling polyethylene terephthalate or PET or polyester, in general three ways have to be differentiated: #The chemical recycling back to the initial raw materials purified terephthalic acid (PTA) or
dimethyl terephthalate Dimethyl terephthalate (DMT) is an organic compound with the formula C6H4(COOCH3)2. It is the diester formed from terephthalic acid and methanol. It is a white solid that melts to give a distillable colourless liquid.Richard J. Sheehan "Terepht ...
(DMT) and
ethylene glycol Ethylene glycol (IUPAC name: ethane-1,2-diol) is an organic compound (a vicinal diol) with the formula . It is mainly used for two purposes, as a raw material in the manufacture of polyester fibers and for antifreeze formulations. It is an odo ...
(EG) where the polymer structure is destroyed completely, or in process intermediates like bis(2-hydroxyethyl) terephthalate #The mechanical recycling where the original polymer properties are being maintained or reconstituted. #The chemical recycling where transesterification takes place and other glycols/polyols or glycerol are added to make a polyol which may be used in other ways such as polyurethane production or PU foam production Chemical recycling of PET will become cost-efficient only applying high capacity recycling lines of more than 50,000 tons/year. Such lines could only be seen, if at all, within the production sites of very large polyester producers. Several attempts of industrial magnitude to establish such chemical recycling plants have been made in the past but without resounding success. Even the promising chemical recycling in Japan has not become an industrial breakthrough so far. The two reasons for this are: at first, the difficulty of consistent and continuous waste bottles sourcing in such a huge amount at one single site, and, at second, the steadily increased prices and price volatility of collected bottles. The prices of baled bottles increased for instance between the years 2000 and 2008 from about 50 Euro/ton to over 500 Euro/ton in 2008. Mechanical recycling or direct circulation of PET in the polymeric state is operated in most diverse variants today. These kinds of processes are typical of small and medium-size industry. Cost-efficiency can already be achieved with plant capacities within a range of 5000–20,000 tons/year. In this case, nearly all kinds of recycled-material feedback into the material circulation are possible today. These diverse recycling processes are being discussed hereafter in detail. Besides chemical contaminants and
degradation Degradation may refer to: Science * Degradation (geology), lowering of a fluvial surface by erosion * Degradation (telecommunications), of an electronic signal * Biodegradation of organic substances by living organisms * Environmental degradatio ...
products generated during first processing and usage, mechanical impurities are representing the main part of quality depreciating impurities in the recycling stream. Recycled materials are increasingly introduced into manufacturing processes, which were originally designed for new materials only. Therefore, efficient sorting, separation and cleaning processes become most important for high quality recycled polyester. When talking about polyester recycling industry, we are concentrating mainly on recycling of PET bottles, which are meanwhile used for all kinds of liquid packaging like water, carbonated soft drinks, juices, beer, sauces, detergents, household chemicals and so on. Bottles are easy to distinguish because of shape and consistency and separate from waste plastic streams either by automatic or by hand-sorting processes. The established polyester recycling industry consists of three major sections: * PET bottle collection and waste separation: waste logistics * Production of clean bottle flakes: flake production * Conversion of PET flakes to final products: flake processing Intermediate product from the first section is baled bottle waste with a PET content greater than 90%. Most common trading form is the bale but also bricked or even loose, pre-cut bottles are common in the market. In the second section, the collected bottles are converted to clean PET bottle flakes. This step can be more or less complex and complicated depending on required final flake quality. During the third step, PET bottle flakes are processed to any kind of products like film, bottles, fiber, filament, strapping or intermediates like pellets for further processing and engineering plastics. Besides this external (post-consumer) polyester bottle recycling, numbers of internal (pre-consumer) recycling processes exist, where the wasted polymer material does not exit the production site to the free market, and instead is reused in the same production circuit. In this way, fiber waste is directly reused to produce fiber, preform waste is directly reused to produce preforms, and film waste is directly reused to produce film.


PET bottle recycling

The only form of PET that is widely recycled in 2022 is the bottle. These are recycled by 'mechanical recycling' increasingly to bottles but still to other forms such as film or fibre. Other forms of polyester are not (as of 2022) collected in significant quantities. Significant investments were announced in 2021 and 2022 for chemical recycling of PET by glycolysis, methanolysis, and enzymatic recycling to recover monomers. Initially these will also use bottles as feedstock but it is expected that fibres will also be recycled this way in future.


See also

*
BoPET BoPET (biaxially-oriented polyethylene terephthalate) is a polyester film made from stretched polyethylene terephthalate (PET) and is used for its high tensile strength, chemical and dimensional stability, transparency, reflectivity, gas and ar ...
(biaxially oriented PET) *
Bioplastic Bioplastics are plastic materials produced from renewable biomass sources, such as vegetable fats and oils, corn starch, straw, woodchips, sawdust, recycled food waste, etc. Some bioplastics are obtained by processing directly from natural bi ...
*
PET bottle recycling Although PET is used in several applications, (principally textile fibres for apparel and upholstery, bottles and other rigid packaging, flexible packaging and electrical and electronic goods), as of 2022 only bottles are collected at a subst ...
*
Plastic recycling Plastic recycling is the reprocessing of plastic waste into new products. When performed correctly, this can reduce dependence on landfill, conserve resources and protect the environment from plastic pollution and greenhouse gas emissions. A ...
* Polycyclohexylenedimethylene terephthalate—a polyester with a similar structure to PET *
Polyester Polyester is a category of polymers that contain the ester functional group in every repeat unit of their main chain. As a specific material, it most commonly refers to a type called polyethylene terephthalate (PET). Polyesters include natural ...
*
Solar water disinfection Solar water disinfection, in short SODIS, is a type of portable water purification that uses solar energy to make biologically-contaminated (e.g. bacteria, viruses, protozoa and worms) water safe to drink. Water contaminated with non-biological age ...
—a method of disinfecting water using only
sunlight Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is scattered and filtered through Earth's atmosphere, and is obvious as daylight when t ...
and
plastic Plastics are a wide range of synthetic or semi-synthetic materials that use polymers as a main ingredient. Their plasticity makes it possible for plastics to be moulded, extruded or pressed into solid objects of various shapes. This adaptab ...
PET bottles


References


External links


American Plastics Council: PlasticInfo.org

KenPlas Industry Ltd.: "What is PET (Polyethylene Terephthalate)"

"WAVE Polymer Technology: PET (Polyethylene Terephthalate) flakes processing"

Arropol commercial producer of polyol from post-consumer PET fiber
{{DEFAULTSORT:Polyethylene Terephthalate Biomaterials Terephthalate esters Commodity chemicals English inventions Flexible electronics Household chemicals Plastics Polyesters Polymers Thermoplastics Transparent materials